Silver nanoparticles dispersed in polymer resin are formulated to make conductive ink which is used for printing of electrical elements such as, electroluminescent (EL) displays, radio frequency identification (RFID), multi-layer circuits and membrane circuit applications. Based on its application, silver ink is printed on different types of media. For example for EL display and RFID applications, it is printed on a porous media. For application of multi-layer circuits and membrane circuit application, it is printed on FR4 board (coated with ink receiving layer) and PET transparency, respectively.
Thermal inkjet printing to which this invention relates is now widely practiced in conventional printing. It involves the intense heating of an aqueous ink in a small amount in contact with a heating element so that the ink is vaporized. The vaporized ink, including solids in the ink, is expelled through a nozzle and thereby directed to an intended substrate.
The objective of formulating silver ink is to provide an ink which has <0.1 ohm per square resistivity, adheres well to media, remains stable up to 1 year shelf life and jets properly in an unmodified black ink cartridge such as Lexmark Black Irk Cartridge #32. Conductive ink with such properties is necessary to produce functional electrical elements.
A previous study by Applicants indicated that at least 11% silver loading is required to obtain <0.1 ohm/sq resistivity after <200° C. sintering process in an oven. The solid loading for conductive ink is very high when compared to traditional pigment or dye inks. To obtain stability, silver dispersion is comprised of finely dispersed silver particles in polymer systems. There are available silver dispersions such as Fine Sphere SVW102 provided by Nippon Paint Co., LTD.
One of the common issues with printed silver ink is its adhesion on the media. Sintered silver traces become powdery and flake off easily from the media. Extra care is needed to handle printed silver traces since they are easily scratched and smeared. The polymer/dispersant in the silver dispersion is not only providing good stability, but sometimes can potentially improve adhesion of the silver particles onto media. However, there is usually a trade off for good adhesion and poor conductivity. As silver particles are dispersed in a higher amount of polymer/dispersant, adhesion is improved but resistivity becomes much worse (i.e., above 0.1 ohm per square).
It would be beneficial to be able to provide high silver loading inks with improved adhesion to media while continuing to meet the <0.1 ohm per square resistivity goal when printed using a commercially available printhead such as Lexmark Black Ink Cartridge #32.